This invention relates to the field of annular-nozzle burners. In particular, this invention concerns a method and apparatus for the introduction of a fuel air mixture into a combustion chamber in a predetermined fuel/air ratio and at a predetermined velocity to obtain high peak temperatures from a compact flame.
In many conventional dryers and burner systems, a solid fuel is mixed with a primary-air carrier and ejected into a combustion chamber. The primary air may be a proportion of the total air required for complete combustion ranging from less than ten percent up to 100 percent. Additional air needed to complete the combustion is then added to the combustion chamber as secondary air. Typically, the secondary air is preheated and enters the combustion chamber at temperatures as high as 1500.degree. F. where it is mixed with the primary air and fuel mixture to complete combustion of the fuel. The primary air-fuel mixture, on the other hand, must be kept below 400.degree. F. to prevent premature combustion or coal dust explosion; and, it is normally kept at or below 180.degree. F.
Existing burner systems such as Deussner et al. (U.S. Pat. No. 4,428,727) and Eckelmann (U.S. Pat. No. 4,373,900) employ the assumption that the maximum temperature obtainable from the combustion flame is limited by the rate of mixing of the fuel with the primary and secondary air. In this respect, existing theory requires that the secondary air be intimately mixed with the fuel/air stream for rapid and complete combustion of the fuel. Thus, current burners have concentrated on methods for inducing rapid convective type mixing of the primary air/fuel with the secondary air.
Existing burner systems also take cognizance of the abrasive wear of pulverized fuel on the burner structure. This results in a controlled fuel stream of relatively low velocity. Additionally, in kiln environments the combustion chambers are often only about 160-200 or so feet long and impingement of the flame on the side walls and far end of the kiln substantially decreases the life of the kiln refractory. Construction of kilns long enough to accommodate the longer flames than have otherwise been desired, on the other hand, has caused a significant increase in the cost of the kiln. Accordingly, lower fuel-air velocities of only about 2500 to 6000 feet per minute have been used to prevent the flame from impinging on the walls of the kilns.
Recent burner developments have also focused on use of deflecting vanes to obtain a spiraling or helical motion to produce a rapid, turbulent mixing of the fuel air stream with the secondary air. Thus, turbulators have been used with and without high velocity air jets to promote such mixing.
The above-described methods have provided excellent mixing of the secondary air with the primary air/fuel stream but, the high temperatures necessary to produce acceptable products in kilns and the like have not always been achieved.
A great deal of effort has also been expended in reducing the amount of the noxious nitrogen oxides (NOx) released from coal burning facilities. The traditional approach to controlling the NOx emissions is to decrease the oxygen concentration and flame temperature by recycling combustion gases. This approach has the disadvantage, however, of increasing the energy consumed in recycling and reheating these recycled gases to the flame temperatures.
In view of the foregoing, it is an object of the instant invention to provide an improved method of burning fuel and an improved burner system to overcome the shortcomings of conventional burners described above.
It is a further object of the instant invention to provide a more efficient annular-nozzle burner that can provide higher peak flame temperatures without damaging the related burner structure or refractory walls.
An alternative object of the invention is to provide an annular-nozzle burner having acceptable NOx levels while reducing the energy-consuming steps of recycling and reheating of exhaust gases.
An advantage of the invention is that it provides a smaller, shorter flame that is essentially anchored to the nozzle so as to permit smaller combustion chambers and reduce the amount of flame impingement on refractory-surface walls. An additional advantage of the invention is an improvement in products that are heated by the burners and method of the invention.
A further advantage of the invention results from an unexpected increase in annulus life even though the invention provides high peak flame temperatures.
A still further advantage of the invention stems from the flame being better stabilized or "anchored" to the burner than those of comparable existing burners. In this respect, where pulverized particulate fuels are employed, it is customary to first use gas or oil-fired flames to heat the refractory walls at a controlled rate until sufficiently hot to maintain combustion of the particulate fuel upon changeover. The instant invention, however provides a core flame which easily stabilizes combustion of the particulate fuel on the burner and permits a more rapid change-over from the more expensive start-up fuels to the desired particulate fuels. In this respect, the following description of the invention will refer to "particles" of fuel. Such use of "particles", however, is not limited to pulverized, particulate fuels, but includes molecules of gas and droplets of liquid fuels as will be clear from the examples herein relating to various fuels. Similarly, although the invention is described in terms of annular-nozzle burners having a cylindrical cross-section, such burners can also have cross-sections that are other than cylindrical.
As a result of much testing and evaluation by the instant inventors, it has been determined that the customary maximizing of convective mixing of secondary air with the primary air-fuel stream has been counterproductive; and, contrary to popular thought, has not provided the most desirable temperatures and flame configurations. Indeed, it is customary for the primary air/fuel mixture to be intensely mixed with secondary air specifically in order to disperse the fuel particles. The instant inventors have found, however, that it is undesirable to increase the average distance between particles.